A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Between the mask and the substrate is disposed a projection system for imaging the irradiated portion of the mask into the target portion of the substrate. The projection system includes components for directing, shaping or controlling the projection beam of irradiation, and these components typically include refractive optics, reflective optics, and/or catadioptric systems, for example.
An important feature in lithography is the size of features of the pattern applied to the substrate. It is desirable to produce apparatus capable of resolving features as small and close together as possible. Furthermore, it is desirable for the maximum resolution available to be uniform across the width of the target portion. The size of the smallest resolvable feature is called the critical dimension (CD). A number of parameters affect the CD at the substrate, and one of the most important of these is the size of features on the mask. The size of feature on the mask leading to the smallest resolvable feature at the substrate gives rise to a critical dimension of the mask. It will be appreciated that, due to the magnification of the projection system, the value of the critical dimension of the mask and the critical dimension (CD) of the substrate may not be the same.
The pattern on a mask is typically produced in a similar manner to that on a wafer. A mask blank comprises a mask substrate (formed from synthetic quartz, for example) coated with a chrome film. A layer of resist is deposited on the chrome film, normally using a spin-on process. The resist is illuminated by a patterned radiation beam, causing chemical changes in the resist in the form of the pattern exposed by the radiation. The mask blank is then developed to fix the pattern in the resist, and then etched so that the pattern is etched into the chrome. The resist is then removed.
The process of depositing resist onto the chrome film has a natural tendency to result in a layer of resist which has a different thickness at the edges of the mask blank than at the center. Some processes result in the resist being thicker at the edges, and some thinner. This has the effect that the critical dimension at the center of the mask blank is different to that at the edge. There is therefore not a uniform critical dimension profile across the width of the mask, and this will lead to a non-uniform CD profile on the substrate.
The ideal solution to this problem would be to modify the deposition of resist so as to produce a substantially flat profile across the full width of the mask. This can be done by adopting a “trial and error” process in setting up the deposition process, which is most accurate when the deposition conditions are substantially identical. Unfortunately it has so far proved impossible to deposit resist with a perfectly flat profile, and in practice resist deposited using such methods, even though almost flat, exhibits a variation in thickness across the mask. Furthermore, this variation in thickness may manifest itself as a “W” shape in the thickness profile, or even as an oscillating thickness. In addition, the thickness profile tends to vary between masks.